Flexor Pollicis Longus, Medial View

Description

Arising from the anterior surface of the radius and the adjacent interosseous membrane, the flexor pollicis longus (FPL) occupies the deep volar compartment of the forearm and is seen from a medial perspective as it courses distal toward the thumb. The muscle belly lies deep to the flexor digitorum superficialis and adjacent to the flexor digitorum profundus, while its tendon passes deep to the flexor retinaculum to enter the hand on the radial side. Distally, the tendon continues along the palmar aspect of the first metacarpal to insert on the base of the distal phalanx of the thumb, crossing the wrist and metacarpophalangeal region en route. Deep anatomy, cleanly oriented. This medial view matters when you need to explain isolated thumb interphalangeal flexion and why loss of FPL function looks different from thenar weakness. Clinically, the tendon’s constrained course through the carpal tunnel and into the thumb makes it susceptible to tenosynovitis and attritional injury, and it is the structure tested by asking the patient to flex the thumb IP joint against resistance (anterior interosseous nerve palsy produces the classic pinch weakness). Surgeons also care about the FPL tendon during volar approaches to the distal radius and during tendon grafting or transfer planning, where maintaining appropriate tendon glide and length-tension is nonnegotiable. Use this asset in upper limb anatomy labs, hand surgery teaching files, or a textbook figure on deep anterior forearm muscles and thumb biomechanics, where a medial view clarifies the tendon’s distal trajectory better than a purely anterior plate. It also suits patient-facing education on carpal tunnel contents and targeted motor testing of the anterior interosseous nerve. Anatomical accuracy verified by SciePro's Medical Advisory Board.